Carbon monoxide (CO) and hydrogen (H2) can be present in air at concentrations of up to about 50 ppm and 10 ppm respectively, although typical concentrations in air are on the order of 1 ppm CO and 1 ppm H2. Normal cryogenic distillation processes used to produce ultra high purity (UHP) nitrogen (N2) do not remove hydrogen and remove only a small portion of the CO. Unless removed by alternative means, these molecules will contaminate the product nitrogen at a concentration up to about two and a half times their concentration in the feed air. Since the electronic industry demands very high purity nitrogen product (typically having on the order of 5 ppb CO or less and 5 ppb H2 or less), CO and H2 have to be removed from feed air.
Air also contains other contaminants such as water (H2O), carbon dioxide (CO2) and hydrocarbons. In cold sections of the distillation separation process (such as heat exchangers and separation columns), water and CO2 can solidify and block the heat exchangers or other parts in the distillation columns. Acetylene and other hydrocarbons in air also present a potential problem because they can accumulate in the liquid oxygen (O2) and create an explosion hazard. It is therefore desirable to remove these impurities prior to the cryogenic distillation of air.
Air prepurification can be accomplished using pressure swing adsorption (PSA), temperature swing adsorption (TSA) or a combination of both (TSA/PSA) incorporating either a single adsorbent or multiple adsorbents. When more than one adsorbent is used, the adsorbents may be configured as discrete layers, as mixtures, composites or combinations of these. Impurities such as H2O and CO2 are commonly removed from air using one or more adsorbent layers in a combined TSA/PSA process. A first layer of activated alumina or zeolite is commonly used for water removal and a second layer of zeolite such as 13X molecular sieve is used for CO2 removal. Prior art, such as U.S. Pat. No. 4,711,645, teaches the use of various adsorbents and methods for removal of CO2 and water vapor from air. These adsorbents are ineffective for the removal of CO and H2, thus allowing CO and H2 to pass through to the distillation equipment.
There are three principal strategies in the prior art to remove CO and/or H2 from air to produce UHP nitrogen: removal upstream of the prepurifier adsorber, removal within the prepurifier adsorber using an oxidation catalyst and removal from the nitrogen product after cryogenic air separation.
In the first approach, CO and H2 are usually removed by high temperature catalytic oxidation over a supported noble metal or hopcalite catalyst upstream of the prepurifier beds. The products from oxidizing CO and H2, namely CO2 and H2O, are removed along with the ambient CO2 and H2O in the prepurifier beds (F. C. Venet, et al., “Understand the Key Issues for High Purity Nitrogen Production,” Chem. Eng. Prog., pp 78-85, January 1993). This approach requires significant power and additional capital, adding substantially to the cost of the process.
U.S. Pat. No. 5,656,557 discloses a process wherein the compressed air is further heated to 350° C. prior to entering a catalyst tower containing palladium (Pd) and/or platinum (Pt) supported catalyst for converting CO, H2 and hydrocarbons to H2O and CO2. The processed air is then cooled to 5° C. to 10° C. prior to entering the prepurifier where the H2O and CO2 are removed. Part of the effluent from the prepurifier may be used as air containing less than 1 ppm total impurities, while the remaining air is separated cryogenically to produce N2 and O2.
French patent FR 2 739 304 describes a method of removing CO and H2 from air which involves; 1) contacting the compressed hot moist gas from the compressor with a bed of CO oxidation catalyst; 2) cooling the resulting intermediate air stream to ambient temperature; 3) contacting this CO free stream with an adsorbent to adsorb CO2 and H2O; and 4) contacting the resulting stream with a H2 trapping adsorbent. The CO catalyst can be copper (Cu) or a Pt group metal supported on alumina, silica or zeolite. The H2 trapping adsorbent can be osmium (Os), iridium (Ir), Pd, ruthenium (Ru), rhodium (Rh) or Pt supported on alumina, silica or zeolite.
U.S. Pat. No. 6,074,621 describes a similar process as FR 2 739 304 except for the cooling step after the CO oxidation catalyst.
U.S. Pat. No. 5,693,302 discloses a method of removing CO and H2 from a composite gas by passing over particles containing gold and Pd supported by TiO2.
U.S. Pat. No. 5,662,873 describes a similar process using a catalyst consisting of silver and at least one element from Pt family supported on alumina, silica or zeolite.
A second technology employed in the prior art is an ambient temperature process for CO and H2 removal from air.
U.S. Pat. No. 5,110,569 discloses a process for removing CO and optionally hydrogen from air by 1) removing water 2) catalytically oxidizing CO to CO2 and optionally H2 to H2O and 3) removing the oxidation products. Oxidation catalysts for CO can be a mixture of manganese and copper oxides such as hopcalite or Carulite. Nickel oxide is also stated to be an effective CO catalyst. The oxidation catalyst for H2 is typically supported palladium.
U.S. Pat. No. 5,238,670 discloses a method of removing CO and/or H2 from air at a temperature between 0° C. and 50° C. by 1) removing water from air until it has a water content lower than 150 ppm and 2) removing CO and H2 on a bed of particles containing at least one metallic element selected from Cu, Ru, Rh, Pd, Os, Ir and Pt deposited by ion-exchange or impregnation on zeolite, alumina or silica.
European patent application EP 0 454 531 describes a similar method which suggests removing both H2O and CO2 prior to the impregnated bed of particles. Traces of H2O and CO2 are removed downstream of the impregnated particle bed.
U.S. Pat. No. 6,048,509 discloses a method for removing CO and H2 from air at ambient temperature wherein air containing H2O, CO2, CO and optionally H2 passes through following steps; 1) contacting the gas with a CO catalyst consisting of Pd or Pt and at least one member selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), Cu, chromium (Cr), tin (Sn), lead (Pb), and cerium (Ce) supported on large pore alumina; 2) contacting the CO free gas with an adsorbent for water removal; 3) contacting the resulting gas with a CO2 adsorbent for CO2 removal and optionally; 4) contacting the gas with a H2 catalyst which consists of Pt or Pd supported on activated alumina or zeolite. The water formed in the last step of hydrogen oxidation is either adsorbed on the H2 catalyst support or it is removed by a H2O adsorbent, which is either physically mixed with the H2 catalyst or placed downstream of it.
U.S. Pat. No. 6,093,379 describes a method where a prepurifier bed with a first layer of water adsorbent and a second layer of CO2 adsorbent operating at ambient temperature is augmented by a third layer of catalyst/adsorbent to remove both CO and H2. The third layer is exposed to substantially H2O-free and CO2-free air at ambient temperature. The dual catalyst/adsorbent is placed in the third layer in the most downstream end of the prepurifier beds. The dual catalyst/adsorbent layer oxidizes CO, adsorbs the resulting CO2, and chemisorbs H2. This dual catalyst/adsorbent is a precious metal such as Pd on a support having a zero point charge (ZPC) of greater than 8.
U.S. Pat. No. 6,511,640 discloses a method wherein a prepurifier is configured to contain various materials layered in series beginning with an adsorbent at the feed inlet for H2O removal. The second layer, an oxidation catalyst to convert CO to CO2, is followed by an adsorbent for CO2 removal. An oxidation catalyst is placed in the next layer to convert H2 to H2O, while the final layer is used for adsorbing H2O. The CO catalyst disclosed is hopcalite, while the H2 catalyst is Pd supported on activated alumina.
A third common strategy for producing UHP N2 in the prior art is the treatment of the cryogenically separated N2 product to remove H2, CO, O2 and other contaminants penetrating the prepurifier and air separation unit.
U.S. Pat. No. 4,579,723 discloses the use of a Ni or Cu supported catalyst or getter to oxidize the contaminants to CO2 and H2O, which are subsequently removed in an adsorber.
European patent EP 0 835 687 teaches regeneration of catalyst beds with a high temperature N2 purge.
Adsorption of CO has been applied in the prior art predominantly for recovery of CO in bulk separations, e.g. in cases where the concentration (partial pressure) of CO is relatively high (typically=1%) and where CO is the more strongly adsorbed component in the gas mixture. Cuprous compounds, either in cationic form in zeolites or dispersed on a porous support, have been widely applied in the recovery of CO from gas mixtures containing CO and N2, methane (CH4), H2 and/or CO2. Materials containing copper in the single oxidation state (denoted as Cu+, Cu(I) or cuprous) display high CO adsorption capacity, while adsorbents containing Cu(II) do not. Adsorbents are commonly synthesized, treated or modified with a Cu(II) compound and then subsequently exposed to a reducing agent such as H2 at elevated temperature to convert the Cu(II) to Cu(I).
Xie et al. (“Highly Efficient Adsorbent for Separation of Carbon Monoxide,” Fundamentals of Adsorption, Proc. IVth Int. Conf. On Fundamentals of Adsorption, Kyoto, May 17-22, 1992, pp. 737-741) describes an adsorbent formed by dispersing CuCl on a zeolite support by mixing the dry powders at elevated temperature. High purity CO separated to high recovery is demonstrated for feed streams containing 9.0% CO/91% N2 and 30.7% CO/65.3% H2/4% CH4.
U.S. Pat. No. 4,917,711 discloses adsorbents and processes utilizing supported CuCl. U.S. Pat. No. 5,531,809 discloses VSA processes using CuCl dispersed on alumina for recovery of CO from synthesis gas exiting a steam-methane reformer.
G. K. Pearce (“The Industrial Practice of Adsorption,” in: Separation of Gases, 5th BOC Priestley Conf., Birmingham, UK Sep. 19-21, 1989, Spec. Publ. No. 80, Royal Soc. Of Chemistry, Cambridge, 1990) provides a description on the use of Cu(I)Y zeolite for the recovery of CO from CO/N2 and CO/H2 feed streams containing percentage (%) levels of CO.
U.S. Pat. No. 4,473,276 discloses Cu(I)Y and Cu-Mordenite along with other exchanged zeolites having a silica to alumina ratio (SiO2/Al2O3)=10 for the recovery of CO.
U.S. Pat. No. 4,019,879 discloses recovery of CO from streams containing H2O and/or CO2 using Cu+ containing zeolites with 20=SiO2/Al2O3=200, e.g. ZSM-5, -8, -11, etc.
Another class of adsorbents having potential for CO adsorption is one in which the materials contain silver (Ag+). Y. Huang (“Adsorption in AgX and AgY Zeolites by Carbon Monoxide and Other Simple Molecules,” J. Catal., 32, pp. 482-491, 1974) provides CO and N2 isotherms for AgX and AgY zeolites to partial pressures only as low as 0.1 to 1.0 torr for the lowest temperature (0° C. and 25° C.) isotherms. The adsorption capacity for CO is significantly greater than that of N2 at 25° C. and 100 torr.
U.S. Pat. No. 4,743,276 discloses mordenite, A, Y and X type zeolites exchanged with various amounts of Ag for the bulk separation (recovery) of CO from refinery and petro-chemical off-gases.
U.S. Pat. No. 4,019,880 relates to the recovery of CO from gas streams containing also H2O and/or CO2 using Ag exchanged zeolites with 20=SiO2/Al2O3=200, e.g. ZSM-5, -8, -11, etc. The invention applies to feed streams containing at least 10 ppm CO at temperatures 0° C.-300° C. The claimed process results in a CO-depleted effluent, e.g. air.
U.S. Pat. No. 4,944,273 discloses zeolites with 1=Si/Al=100 and doped with Ca, Co, Ni, Fe, Cu, Ag, Pt or Ru for adsorption of oxides of nitrogen (NOx) and CO as part of NOx and CO sensors, particularly in exhaust gases of automotive vehicles.
U.S. Pat. No. 3,789,106 discloses that mordenite charged with copper is effective in removing CO from H2 at CO partial pressure below 3 mmHg. The effectiveness was determined by subjecting the adsorbent to CO concentrations greater than or equal to 100 ppm and measuring capacity at saturation.
The above prior art relating to adsorption of CO is almost totally silent with respect to purification of CO from mixed gas streams, particularly those containing less than 10 ppm CO in O2 and N2.